Molecular models are widely used by practicing chemists, and by students and teachers of chemistry. One reaction mechanism that widely occurs is the Walden inversion, also known as atomic inversion or stereochemical inversion. This is a reaction mechanism that obtains for a great many chemical reactions, most of which are known as bimolecular nucleophilic substitution (S.sub.N 2) reactions. This reaction mechanism must be learned by every student of organic chemistry and is widely described in textbooks on the subject, for example, Roberts & Caserio, Basic Principles of Organic Chemistry, 2d Ed., W. A. Benjamin (pub.), Menlo Park, Calif. (1977).
S.sub.N 2 reactions, as well as various models for exhibiting these reactions, have been disclosed in, among others, the following articles: Noller, "A Simple Apparatus to Demonstrate Walden Inversion", 24 J. Chem. Educ. 277 (June 1947); Nyquist, "A Simple Model for the S.sub.N 2 Mechanism", 42 J. Chem. Educ. 103 (February 1965); Hamon, "A Model to Demonstrate the Walden Inversion", 47 J. Chem. Educ. 398 (May 1970); Sone and Sone, "S.sub.N 1 and S.sub.N 2 Reactions: Paper Marionette for Demonstration", 50 J. Chem. Educ. 615 (September 1973); NeWman, "A Molecular Model for S.sub.N 2 Reactions", 52 J. Chem. Educ. 462 (July 1975); Anderson, "Two Working Models for the S.sub.N 2 Mechanism", 64 J. Chem. Educ. 1023 (December 1987); Berry and Botterill, "A Molecular Model to Demonstrate Walden Inversion", 4 Educ. Chem. 139 (May 1967).
When four atoms are attached to a central carbon or other atom with single bonds, they normally assume a tetrahedral configuration around the central atom. During a Walden inversion, a fifth atom attaches itself to the central atom opposite one of the four single-bonded atoms, displacing the one atom. At the same time, the remaining three original atoms rotate away from the new, fifth atom to establish a tetrahedral configuration with respect to the fifth atom.
One of the significant disadvantages inherent in all known previous molecular models is that such inversions could be demonstrated with respect to only one axis. Therefore, it was impossible to demonstrate, with a single model, sequential inversions which occur along different axes around the central atom without disassembling and reconstructing the model so as to be properly oriented around the next inversion axis.